Hens are older than 12 months when they get kicked out as they don't got into the units until they are point of lay at about 20 weeks,I've never had any that didn't lay and the ones i've got now still lay like engines a year later.
Try not to give them too much hard feed initially like grit,pellets or even long grass as they've basically been living off powdered feed for so long and the gizzard is a muscle that needs time to work as it should do naturally and grind food prior to digestion Other than that you shouldn't have any problems aslong as you don't get any red mite in the shed
cheer